Bipolar transistor having collector with grading

ABSTRACT

This disclosure relates to bipolar transistors, such as heterojunction bipolar transistors, having at least one grading in the collector. One aspect of this disclosure is a bipolar transistor that includes a collector having a high doping concentration at a junction with the base and at least one grading in which doping concentration increases away from the base. In some embodiments, the high doping concentration can be at least about 3×10 16  cm −3 . According to certain embodiments, the collector includes two gradings. Such bipolar transistors can be implemented, for example, in power amplifiers.

BACKGROUND

1. Technical Field

The present disclosure relates to the field of semiconductor structuresand, more particularly, to bipolar transistors and products that includebipolar transistors.

2. Description of the Related Technology

Bipolar transistors, such as heterojunction bipolar transistors (HBTs),are implemented in a wide variety of applications. Such bipolartransistors can be formed on semiconductor substrates, such as galliumarsenide (GaAs) substrates. One illustrative application for a bipolartransistor is in a power amplifier system. As technology evolves,specifications for power amplifier systems have become more demanding tomeet.

One aspect of power amplifier performance is linearity. Measures oflinearity performance can include channel power ratios, such as anadjacent channel power ratio (ACPR1) and an alternative channel powerratio (ACPR2), and/or channel leakage power ratios, such as an adjacentchannel leakage power ratio (ACLR1) and an alternative channel leakagepower ratio (ACLR2). ACPR2 and ACLR2 can be referred to as secondchannel linearity measures. ACPR2 and ACLR2 values can correspond atmeasurements at an offset of about 1.98 MHz from a frequency ofinterest.

Conventionally, most publications have focused on ACPR1 and ACLR1linearity measures and little has been published about ACRP2 or ACLR2.Recent ACPR2 and ACLR2 system specifications have been particularlydifficult to meet, especially while meeting other system specificationsrelated to RF gain. Accordingly, a need exists for improved linearity insystems that include bipolar transistors, such as power amplifiersystems.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

The innovations described in the claims each have several aspects, nosingle one of which is solely responsible for its desirable attributes.Without limiting the scope of this invention, some prominent featureswill now be briefly discussed.

One aspect of this disclosure is a bipolar transistor that includes acollector, a base disposed over the collector, and an emitter. Thecollector has a doping concentration of at least about 3×10¹⁶ cm⁻³ in afirst collector region abutting the base. The collector also has another collector region under the first collector region. The othercollector region includes at least one grading in which dopingconcentration increases away from the first collector region.

In certain embodiments, the other collector region includes a firstgrading and a second grading in which doping concentration increasesaway from the base at a different rate than in the first grading.According to some of these embodiments, the bipolar transistor of canhave a gain of at least about 29 dBm at a frequency within a frequencyband centered around about 833 MHz. In accordance with a number ofembodiments, the second grading of the bipolar transistor can beconfigured to increase Bv_(CEX) of the bipolar transistor compared tothe same transistor without the second grading at the same currentdensity. In various embodiments, a doping concentration in the firstgrading grades from about an order of magnitude less than the dopingconcentration of the first collector region to less than the dopingconcentration of the first collector region. According to some of theseembodiments, a doping concentration in the second grading grades fromabout a maximum doping concentration in the first grading to a dopingconcentration that is at least about one order of magnitude less thanthe doping concentration of a sub-collector below the second grading. Insome embodiments, the first grading spans a second collector regionproximate the first collector region and having a thickness that is morethan approximately twice the thickness of the first collector region.According to certain embodiments, the second grading spans a thirdcollector region having a thickness that is greater than the thicknessof the first collector region and less than the thickness of the secondcollector region. In various embodiments, the collector consistsessentially of the first collector region, the second collector region,and the third collector region. According to some embodiments, thebipolar transistor also includes a sub-collector under the collector. Inaccordance with certain embodiments, the first grading borders thesecond grading and doping concentration is approximately the same onboth sides of the border of the first grading and the second grading.

In certain embodiments, a thickness of the first collector region isselected from a range of about 1000 Å to 2000 Å. According to some ofthese embodiments, the doping concentration of the first collectorregion is selected from a range of about 3×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³.

According to a number of embodiments, the doping concentration in thefirst collector region is at least about 6×10¹⁶ cm⁻³.

In accordance with some embodiments, the base has a thickness of lessthan about 1400 Å. In some of these embodiments, the base has a dopingconcentration selected from a range of about 3.5×10¹⁹ cm⁻³ to 7×10¹⁹cm⁻³.

In a number of embodiments, the bipolar transistor is a heterojunctionbipolar transistor (HBT).

According to some embodiments, the bipolar transistor is a GaAstransistor.

Another aspect of this disclosure is a power amplifier module thatincludes a bipolar transistor. The bipolar transistor has a collector, abase, and an emitter. The collector has a doping concentration at ajunction with the base such that the power amplifier has an alternativechannel power ratio (ACPR2) of no greater than about −65 dBc. Thecollector also has at least a first grading in which dopingconcentration increases away from the base.

According to certain embodiments, the ACPR2 is no greater than about −65dBc when the power amplifier operates within a frequency band centeredaround approximately 833 MHz.

In a number of embodiments, the collector also includes a second gradingfarther from the base than the first grading. The second grading isconfigured to increase Bv_(CEX) of the bipolar transistor compared tothe same transistor without the second grading at the same currentdensity, according to some embodiments.

In accordance with a number of embodiments, the doping concentration inthe collector at the junction with the base is at least about 3×10¹⁶cm⁻³.

In certain embodiments, the collector includes a first region abuttingthe base having a substantially flat doping concentration of at leastabout 3×10¹⁶ cm⁻³ and a thickness selected from a range of about 1000 Åto 2000 Å. According to some of these embodiments, the dopingconcentration in the first region of the collector is selected in therange from about 3×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³.

Another aspect of this disclosure is a power amplifier die that includesa bipolar transistor having a collector, a base abutting the collector,and an emitter. The collector has a doping concentration of at leastabout 3×10¹⁶ cm⁻³ at a junction with the base. The collector also has atleast a first grading in which doping concentration increases away fromthe base.

Another aspect of this disclosure is a mobile device that includes anantenna, a battery, and a power amplifier. The power amplifier includesa heterojunction bipolar transistor having a collector, a base, and anemitter. The collector includes a first collector region abutting thebase and having a first doping concentration of at least about 3×10¹⁶cm⁻³. The collector also includes a second collector region proximatethe first collector region and having a first grading in which dopingconcentration increases away from the base. The collector also includesa third collector region proximate the second collector region andhaving a second grading in which doping concentration increases awayfrom the base at a different rate than the first grading. The firstdoping concentration, the first grading, and the second grading areconfigured to improve linearity of the power amplifier.

Yet another aspect of this disclosure is a method of forming a bipolartransistor. The method includes forming a sub-collector; forming acollector region with at least one grading having a doping concentrationthat decreases away from the sub-collector; and forming a differentcollector region adjacent abutting a base of the bipolar transistor andhaving a doping concentration of at least about 3×10¹⁶ cm⁻³ at aninterface with the base.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the inventions have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment of the invention.Thus, the invention may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts an illustrative cross section of a bipolar transistoraccording to an embodiment.

FIG. 1B is a graph of example doping concentrations of portions of thebipolar transistor of FIG. 1A.

FIG. 1C is a legend illustrating example materials corresponding toportions of the bipolar transistor of FIG. 1A.

FIG. 2 is a graph that illustrates relationships between breakdownvoltage and current density for the bipolar transistor of FIG. 1A and astate of the art bipolar transistor.

FIG. 3A depicts an illustrative cross section of a bipolar transistoraccording to another embodiment.

FIG. 3B is a graph of example doping concentrations of portions of thebipolar transistor of FIG. 3A.

FIG. 3C is a legend illustrating example materials corresponding toportions of the bipolar transistor of FIG. 3A.

FIG. 3D depicts an illustrative cross section of a bipolar transistoraccording to another embodiment.

FIG. 3E is a graph of example doping concentrations of portions of thebipolar transistor of FIG. 3D.

FIG. 3F is a legend illustrating example materials corresponding toportions of the bipolar transistor of FIG. 3D.

FIG. 4 is an illustrative flow diagram of making a bipolar transistoraccording to an embodiment.

FIG. 5 is an illustrative block diagram of a power amplifier module thatincludes a bipolar transistor with one or more features describedherein.

FIG. 6 is an illustrative block diagram of a mobile device that includesthe power amplifier module of FIG. 5.

DETAILED DESCRIPTION

Generally described, aspects of the present disclosure relate to abipolar transistor having a high doping concentration (for example, atleast about 3×10¹⁶ cm⁻³) in a first collector region abutting a base andat least one grading in another collector region adjacent the firstcollector region. A high doping concentration in a first collectorregion abutting a base of the bipolar transistor can improve secondchannel linearity measures, such as ACPR2 and/or ACLR2, in poweramplifier systems. However, the high doping concentration in the firstcollector region can also decrease a gain of the bipolar transistor,such as the RF gain. To offset the decrease in the gain resulting fromthe high doping concentration in the first collector region, one or moregradings can be included in the other collector region to transitionfrom the high doping concentration in the first collector region to asub-collector. In some embodiments, the other collector region includestwo different gradings in which doping concentration varies (forexample, increases) at different rates away from the base. Properlyselecting the grading(s) and the doping concentration in the firstcollector region can result in desirable RF gain and ruggednesscharacteristics of the bipolar transistor, especially compared to if thebipolar transistor included a flat doped or step doped collectorstructure.

Experimental data indicate that power amplifier systems that includesuch bipolar transistors can meet demanding second channel linearityspecifications and also meet RF gain specifications. For instance, apower amplifier system including such a bipolar transistor can have anACPR2 of no greater than about −65 dBc and a gain of at least about 29dBm when operating at a frequency within a frequency band centeredaround approximately 833 MHz. In contrast, purely circuit designtechniques that have been attempted to achieve desired levels of ACPR2or ACLR2 have had limited success. Moreover, other bipolar transistorswith enhanced ACPR2 and/or ACLR2 had degraded RF gain.

FIG. 1A shows an illustrative cross section of a bipolar transistor 100according to an embodiment. As illustrated, the bipolar transistor 100is a heterojunction bipolar transistor (HBT). The bipolar transistor 100can be formed on a substrate 106. The substrate 106 can be asemiconductor substrate, such as a GaAs substrate. The bipolartransistor 100 can be disposed between isolation regions 110 and 112.Isolation regions 110 and 112 are non-conductive regions that canprovide electrical isolation between the bipolar transistor 100 and anadjacent transistor or other circuit element. Isolations regions 110 and112 can each include, for example, a trench filled with nitride,polyimide, or other material suitable for electrical isolation. Althoughnot shown, it will be understood that one or more buffer layers can beincluded between the substrate 106 and the sub-collector 108. The one ormore buffer layers can include implant damaged material that renderssuch material semi-insulating.

The bipolar transistor 100 can include a collector 120, a base 121, andan emitter 128. The collector 120 can include a plurality of collectionregions having different doping profiles. For instance, the collector120 can include a first collector region 122 abutting the base 121 andan other collector region 125 that includes at least one grading inwhich doping concentration increases away from the first collectorregion 121. As illustrated in FIG. 1A, the other collector region 125can include a second collector region 123 under the first collectorregion 122 and a third collector region 124 under the second collectorregion 123.

The first collector region 122 can abut the base 121 to form acollector-base junction. The collector-base junction can be a p-njunction. The first collector region 122 can include N+ doped GaAs. Thefirst collector region 122 can be a flat doped region. Thus, within thefirst collector region 122, the doping concentration can besubstantially constant. The doping concentration in the first collectorregion 122 at the collector-base interface of the bipolar transistor 100can influence linearity of a system that includes the bipolar transistor100. For instance, the doping concentration of the first collectorregion 122 together with the thickness of the first collector region 122can influence ACPR2 and/or ACLR2 of a power amplifier system. Lowerdoping concentrations of the first collector region 122 together withsmaller thickness of the first collector region 122 may not achieve adesired level of ACPR2 and/or ACLR2. On the other hand, higher dopingconcentrations of the first collector region 122 together with largerthickness of the first collector region 122 may degrade a gain of thebipolar transistor 100 such that a system including the bipolartransistor 100 does not meet gain specifications, such as RF gainspecifications. In view of this trade-off, particular values of thedoping concentration of the first collector region 122 and the thicknessof the first collector region 122 may need to be selected to achieveboth a desired gain and a desired linearity. As one example, for a GaAsbipolar transistor 100, FIG. 1B indicates that the first collectorregion 122 has a doping concentration of 6×10¹⁶ cm⁻³ and a thickness of2000 Å.

The first collector region 122 can have a doping concentration that isselected to meet ACPR2 and/or ACLR2 specifications of a power amplifiersystem that includes the bipolar transistor 100. As one example, thefirst collector region 122 can have a doping concentration selected suchthat the a system that includes the bipolar transistor 100 has an ACPR2of no greater than about −65 dBc and a gain of at least about 29 dBmwhen operating at a frequency within a frequency band centered aroundapproximately 833 MHz. In some embodiments, the first collector region122 can have a doping concentration selected such that the a system thatincludes the bipolar transistor 100 has an ACPR2 of no greater thanabout −55 dBc, no greater than about −57 dBc, no greater than about −60dBc, no greater than about −62 dBc, no greater than about −65 dBc, nogreater than about −67 dBc, no greater than about −70 dBc, no greaterthan about −72 dBc, or no greater than about −75 dBc. These values ofACPR2 can hold for an entire range of output power of the system and/orfor one or more frequency bands of operation within the RF frequencyrange. As one example, to meet some ACPR2 and/or ACLR2 specifications,the first collector region 122 can have a doping concentration of atleast about 3×10¹⁶ cm⁻³.

In some embodiments, the first collector region 122 can have a dopingconcentration of at least about 3×10¹⁶ cm⁻³, at least about 3.5×10¹⁶cm⁻³, at least about 4×10¹⁶ cm⁻³, at least about 4.5×10¹⁶ cm⁻³, at leastabout 5×10¹⁶ cm⁻³, at least about 5.5×10¹⁶ cm⁻³, at least about 6×10¹⁶cm⁻³, at least about 6.5×10¹⁶ cm⁻³, at least about 7×10¹⁶ cm⁻³, at leastabout 7.5×10¹⁶ cm⁻³, at least about 8×10¹⁶ cm⁻³, at least about 8.5×10¹⁶cm⁻³, or at least about 9×10¹⁶ cm⁻³. According to certain embodiments,the first collector region 122 can have a doping concentration selectedwithin one of the following ranges: about 3×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³,about 3×10¹⁶ cm⁻³ to 8×10¹⁶ cm⁻³, about 3×10¹⁶ cm⁻³ to 7×10¹⁶ cm⁻³,about 3×10¹⁶ cm⁻³ to 6×10¹⁶ cm⁻³, about 3×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³,about 4×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³, about 4×10¹⁶ cm⁻³ to 8×10¹⁶ cm⁻³,about 4×10¹⁶ cm⁻³ to 7×10¹⁶ cm⁻³, about 4×10¹⁶ cm⁻³ to 6×10¹⁶ cm⁻³,about 4×10¹⁶ cm⁻³ to 5×10¹⁶ cm⁻³, about 5×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³,about 5×10¹⁶ cm⁻³ to 8×10¹⁶ cm⁻³, about 5×10¹⁶ cm⁻³ to 7×10¹⁶ cm⁻³,about 5×10¹⁶ cm⁻³ to 6×10¹⁶ cm⁻³, about 6×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³,about 6×10¹⁶ cm⁻³ to 8×10¹⁶ cm⁻³, about 6×10¹⁶ cm⁻³ to 7×10¹⁶ cm⁻³,about 7×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³, about 7×10¹⁶ cm⁻³ to 8×10¹⁶ cm⁻³, orabout 8×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³.

The thickness of the first collector region 122 can be selected in therange from about 500 Å to 4000 Å in accordance with certain embodiments.In some of these embodiments, the thickness of the first collectorregion 122 can be selected within one of the following ranges: about 500Å to 1000 Å, about 1000 Å to 2000 Å, about 1000 Å to 3000 Å, about 1500Å to 2000 Å, about 2000 Å to 3000 Å, about 2000 Å to 4000 Å, about 2500Å to 4000 Å, or about 3000 Å to 4000 Å. Any of these thickness rangescan be implemented in combination with any of the doping concentrationsdiscussed earlier. In the bipolar transistor 100 of FIG. 1A, thethickness of the first collector region 122 can be measured as ashortest distance between the base 121 and the other collector region125.

Higher doping concentrations in the first collector region 122 canreduce the RF gain of the bipolar transistor 100. In order to meet RFgain specifications of a system that includes the bipolar transistor100, such as a power amplifier system, other changes to features of thebipolar transistor 100 may need to counteract such a decrease in RFgain. One or more gradings in the other collector region 125 of thebipolar transistor 100 can compensate for some or all of the losses inRF gain associated with a higher doping concentration in the firstcollector region 122. At the same time, ACPR2 and/or ACLR2specifications of a power amplifier system that includes the bipolartransistor 100 can still be met.

The other collector region 125 can include multiple gradings in whichdoping varies at different rates. As illustrated in FIGS. 1A and 1B, theother collector region 125 can include a second collector region 123having the first grading and a third collector region 124 having thesecond grading. In the first grading, the doping concentration canincrease in a direction away from the base 121. The doping concentrationcan also increase in a direction away from the base 121 in the secondgrading. The doping concentration can increase at a different rate inthe second grading than in the first grading. For instance, asillustrated in FIG. 1B, the doping concentration can increase at agreater rate in the second grading than in the first grading. In someother implementations, the first grading and the second grading can haverespective doping concentrations that increase at substantially the samerate. For instance, there can be a discontinuity in doping concentrationwhere the collector transitions from the first grading to the secondgrading and/or there can be a collector region with a flat dopingbetween the first grading and the second grading. The first gradingand/or the second grading can vary linearly or non-linearly (forexample, parabolically). In the example illustrated in FIG. 1B, thefirst grading and the second grading can both have doping concentrationsthat vary linearly.

The second collector region 123 can include N− doped GaAs. The firstgrading can span the second collector region 123. The dopingconcentration in the second collector region 123 can increase away fromthe base 121 and the first collector region 122. In some embodiments,the doping concentration of the second collector region 123 adjacent thefirst collector region 122 can begin at a doping concentration that isabout one order of magnitude lower than the doping concentration of thefirst collector region 121. For example, as shown in FIG. 1B, the dopingconcentration of the first collector region 121 can be about 6×10¹⁶ cm⁻³and the lowest doping concentration of the second collector region canbe about 7.5×10¹⁵ cm⁻³. As also shown in FIG. 1B, the second collectorregion 123 can have a thickness of about 5000 Å and the dopingconcentration can grade from about 7.5×10¹⁵ cm⁻³ at an interface withthe first collector region 121 to 3×10¹⁶ cm⁻³ at an interface with thethird collector region 124. In some embodiments, the dopingconcentration at the interface with the third collector region 124 canbe substantially the same where the first grading meets the secondgrading. This can reduce discontinuities in capacitance associated withthe collector 120. The first grading can reduce base to collectorcapacitance and consequently increase a gain, such as an RF gain, of thebipolar transistor 100.

The third collector region 124 can include N− doped GaAs. The secondgrading can span the third collector region 124. The dopingconcentration in the third collector region 124 can increase away fromthe second collector region 123. The doping concentration of the thirdcollector region 124 adjacent the second collector region 123 can have adoping concentration that is approximately equal to the maximum dopingconcentration of the second collector region 123. As also shown in FIG.1B, the second collector region 123 can have a thickness of about 3000 Åand the doping concentration can grade from about 3×10¹⁶ cm⁻³ at aninterface with the second collector region 123 to 6×10¹⁶ cm⁻³ at aninterface with the sub-collector 108. In some embodiments, the maximumdoping concentration of the third collector region 124 can be about twoorders of magnitude lower than the doping concentration of thesub-collector 108. For example, as shown in FIG. 1B, the maximum dopingconcentration of the third collector region 124 can be about 6×10¹⁶ cm⁻³and the doping concentration of the sub-collector 108 can be about5×10¹⁸ cm⁻³.

The doping concentration of the third collector region 124 at aninterface with the sub-collector 108 can determine a breakdown voltagefrom collector to emitter with the base having a resistor coupled to apotential. Such a breakdown voltage can be referred to as “BV_(CEX).” Ahigher BV_(CEX) can increase a safe operating region (SOA). Higherdoping in the third collector region 124 at the interface with thesub-collector 108 can reduce the SOA. Doping the third collector region124 at the interface with the sub-collector 108 too low can result in abreakdown current that is too steep, thereby reducing robustness of thebipolar transistor 100. In certain embodiments, the doping concentrationin the third collector region 124 at the interface with thesub-collector 108 can be selected in the range from about 5×10¹⁶ cm⁻³⁶to 9×10¹⁶ cm⁻³. Such doping concentrations can result in desirableBV_(CEX) values for the bipolar transistor 100 and/or a desirable SOA.More detail regarding BV_(CEX) values associated with the bipolartransistor 100 will be provided with reference to FIG. 2.

The base 121 can include P+ doped GaAs. The base 121 can be thinnerand/or have a higher doping concentration than bases in other bipolartransistors used in power amplifier systems. Reducing the thickness ofthe base 121 and increasing the doping concentration of the base 121 canincrease the RF gain and keep the DC gain substantially the same. Forexample, in certain implementations, the doping concentration of thebase 121 can be selected in a range from about 2×10¹⁹ cm⁻³ to 7×10¹⁹cm⁻³. The thickness of the base 121 can be selected in the range fromabout 350 Å to 1400 Å according to certain implementations. In someimplementations, the thickness of the base 121 can be selected in therange from about 500 Å to 900 Å. Any base thicknesses selected from theranges disclosed herein can be implemented in combination with any ofthe base doping concentrations selected from the ranges disclosedherein. As one example, the base 121 can have a doping concentration of5.5×10¹⁹ cm⁻³ and a thickness of 500 Å. In the bipolar transistor 100 ofFIG. 1A, thickness can be the shortest distance between the emitter 128and the first collector region 121.

The product of the doping and the thickness of the base 121 can bereferred to as a “Gummel number.” In some embodiments, the Gummel numbercan be approximately constant such that the bipolar transistor 100 canhave an approximately constant beta value. For example, increasing thethickness of the base 121 within a selected range can be accompanied bya corresponding decrease in doping concentration of the base 121 to holdthe Gummel number approximately constant. As another example, decreasingthe thickness of the base 121 within a selected range can be accompaniedby a corresponding increase in doping concentration of the base 121 tohold the Gummel number approximately constant. Reducing the thickness ofthe base 121 and increasing the doing of the base 121 can result ininsignificant changes in resistance associated with the base 121. Forinstance, changing the thickness of the base 121 from 900 Å to 500 Å andchanging the doping concentration of the base 121 from 4×10¹⁹ cm⁻³ to5.5×10¹⁹ cm⁻³ may not have a significant effect on resistance of thebase 121.

The bipolar transistor 100 can include a collector contact 136 to thecollector, base contact(s) 138 to the base 121, and an emitter contact142 to the emitter 126. These contacts can provide an electricalconnection to and/or from the bipolar transistor 100. The contacts 136,138, and 142 can be formed of any suitable conductive material. Asillustrated in FIG. 1A, the emitter contact 142 can be disposed over atop contact 134, a bottom contact 132, and an emitter cap 126.

The bipolar transistor 100 can include a sub-collector 120 over thesubstrate 106. The sub-collector 120 can be under the other collectorregion 125. For example, as illustrated in FIG. 1A, the sub-collector120 can be disposed between the third collector region 124 and thesubstrate 108. The sub-collector 120 can abut the third collector region124. The sub-collector 120 can be a flat doped region. In someembodiments, the doping concentration of the sub-collector 120 can be atleast one or two orders of magnitude higher than the highest dopingconcentration of the third collector region 124. As shown in FIG. 1B,the sub-collector 120 can have a doping concentration on the order of5×10¹⁸ cm⁻³ and have a thickness of at least about 8000 Å in certainembodiments. The collector contact 136 physically contacting thesub-collector 120 can provide an electrical connection to the collector120.

FIG. 1C is a legend 150 illustrating example materials corresponding toportions of the bipolar transistor 100 of FIG. 1A. Dashed lines betweenFIG. 1A and FIG. 1C are included to indicate that materials in thelegend 150 correspond to particular portions of the bipolar transistor100. The legend 150 indicates that, in certain embodiments, thesubstrate 108 can be semi-insulating GaAs, the sub-collector 120 can beN+ GaAs, the third collector region 124 can be N− GaAs, the secondcollector region 123 can be N− GaAs, the first collector region 122 casnbe N+ GaAs, the base 121 can be P+ GaAs, the emitter 128 can be N−InGaP, the emitter cap 126 can be N− GaAs, the bottom contact 132 can beN+ GaAs, and the top contact 134 can be InGaAs. It will be understoodthat in some embodiments, one or more of the regions of the bipolartransistor 100 can include a suitable alternative material instead ofthe example materials provided in the legend 150. Moreover, in any ofthe bipolar transistors described herein n-type doping and p-type dopingcan be interchanged throughout some or all of the transistor. Thus, anycombination of features described herein can be applied to NPNtransistors and/or PNP transistors.

Experimental data indicate that a power amplifier system including thebipolar transistor 100 of FIG. 1A has met currently linearityspecifications, including ACPR2 and ACLR2, and RF gain specificationsthat have been particularly challenging to meet. Moreover, experimentaldata indicate that the bipolar transistor 100 of FIG. 1A has desirableruggedness qualities, for example, as indicated by BV_(CEX) values andthe safe operating region (SOA).

FIG. 2 is a graph that illustrates relationships between BV_(CEX) andcurrent density for the bipolar transistor 100 of FIG. 1A and aconventional bipolar transistor. In FIG. 2, “+” symbols represent datacorresponding to the bipolar transistor 100 and “o” symbols representdata corresponding to a current, state of the art bipolar transistor. Asmentioned earlier, BV_(CEX) can represent a breakdown voltage fromcollector to emitter in a bipolar transistor with the base having aresistor coupled to a potential.

In FIG. 2, the SOA is represented by the area below the illustratedBV_(CEX) curves. When a bipolar transistor operates at a voltage andcurrent density corresponding to its BV_(CEX) curve, the bipolartransistor reaches a point at which it breaks down. Moreover, when abipolar transistor operates at a voltage and current density that areabove its corresponding BV_(CEX) curve, the bipolar transistor breaksdown.

The data in FIG. 2 indicate that the bipolar transistor 100 operateswithin the SOA when operating at voltages below a BV_(CEX) value on thecorresponding BV_(CEX) curve at a particular current density. The datain FIG. 2 also indicate that the bipolar transistor 100 operates withinthe SOA when operating at current densities below the current density onthe corresponding BV_(CEX) at particular voltage level. Further, so longas a voltage and current density combination is below the BV_(CEX)curve, the bipolar transistor should operate within the SOA. As shown inFIG. 2, the bipolar transistor 100 has a larger SOA than theconventional bipolar transistor. The bipolar transistor 100 hasincreased ruggedness compared to the conventional bipolar transistorbecause it has a larger SOA and can operate at higher current densitiesand voltages without breaking down. Thus, the bipolar transistor 100 hasdesirable ruggedness characteristics.

FIG. 3A depicts an illustrative cross section of a bipolar transistor300A according to another embodiment. The bipolar transistor 300A ofFIG. 3A is substantially the same as the bipolar transistor 100 of FIG.1A except the other collector region 325 of FIG. 3A is different fromthe other collector region 125 of FIG. 1A. More specifically, the othercollector region 325 has a different doping profile than the othercollector region 125. FIG. 3B is a graph that shows illustrative dopingconcentrations of portions of the bipolar transistor 300A of FIG. 3A.

The bipolar transistor 300A can include a collector 120 having a firstcollector region 122 and an other collector region 325. The firstcollector region 122 can include any combination of features describedwith reference to the first collector region 122 of FIG. 1A. The othercollector region 325 can include a single grading in which dopingconcentration varies (for example, increases) away from the base 121.

In order to meet RF gain specifications of a system, such as a poweramplifier system, that includes the bipolar transistor 300A, the singlegrading in the other collector region 325 of the bipolar transistor 300Acan compensate for some or all of the losses in RF gain associated witha higher doping concentration in the first collector region 122. At thesame time, ACPR2 and/or ACLR2 specifications of a power amplifier systemthat includes the bipolar transistor 300A can still be met. The othercollector region 325 can include a second collector region 323 and athird collector region 324 as illustrated in FIGS. 3A and 3B. In otherembodiments, for example, as shown in FIGS. 3D-3F, the flat dopedportion can be omitted from the other collector region 325.

As illustrated in FIGS. 3A and 3B, the other collector region 325 caninclude a second collector region 323 having a flat doping. The secondcollector region 323 can include N− doped GaAs. In some embodiments, thedoping concentration of the second collector region 323 has at a dopingconcentration that is about one order of magnitude lower than the dopingconcentration of the first collector region 121. According to certainembodiments, the doping concentration of the second collector region canbe selected from the range of about 7.5×10¹⁵ cm⁻³ to 1.5×10¹⁶ cm⁻³. Thesecond collector region 323 can have a thickness selected from the rangefrom about 2000 Å to 4000 Å. In some embodiments, the dopingconcentration of the second collector region 323 can be approximatelyequal to the doping concentration at which the third collector region324 begins to grade. This can reduce discontinuities in capacitanceassociated with the collector 120.

The third collector region 324 can include N− doped GaAs. The singlegrading can span the third collector region 324. In other embodiments,for example, as shown in FIGS. 3D-3F, the single grading can span theother collector region 335. The doping concentration in the thirdcollector region 324 can increase away from the base 121, the firstcollector region 121, and/or the second collector region 323. The dopingconcentration of the third collector region 324 adjacent the secondcollector region 323 can have a doping concentration that isapproximately equal to the doping concentration of the second collectorregion 323. The third collector region 324 can have a thickness selectedfrom the range from about 4000 Å to 7000 Å. The doping concentration inthe third collector region 324 can grade from about 7.5×10¹⁵ cm⁻³ at aninterface with the second collector region 323 to at least about 5×10¹⁶cm⁻³ at an interface with the sub-collector 108. In some embodiments,the maximum doping concentration of the third collector region 324 canbe about two orders of magnitude lower than the doping concentration ofthe sub-collector 108.

The doping concentration of the third collector region 324 at aninterface with the sub-collector 108 can determine BV_(CEX). Higherdoping in the third collector region 324 at the interface with thesub-collector 108 can reduce the SOA. Doping the third collector region324 at the interface with the sub-collector 108 too low can result in abreakdown current that is too steep, thereby reducing robustness of thebipolar transistor 300A. In certain embodiments, the dopingconcentration in the third collector region 324 at the interface withthe sub-collector 108 can be selected in the range from about 5×10¹⁶cm⁻³ to 9×10¹⁶ cm⁻³. Such doping concentrations can result in desirableBV_(CEX) values for the bipolar transistor 300A and/or a desirable SOA.

As shown in the legend 150 of FIG. 3C, the bipolar transistor 300A canbe formed of substantially the same materials as the bipolar transistor100, with a different doping profile in the collector 120.

FIG. 3D depicts an illustrative cross section of a bipolar transistor300B according to another embodiment. The bipolar transistor 300B ofFIG. 3D is substantially the same as the bipolar transistor 300A of FIG.3A except the other collector region 335 of FIG. 3D is different fromthe other collector region 325 of FIG. 3A. More specifically, a gradingspans the other collector region 335. The collector 120 of the bipolartransistor 300B can consist of the first collector region 122 and theother collector region 335. As illustrated in FIG. 3D, the collector 120of the bipolar transistor 300B only includes the first collector region122 and the second other collector region 335. FIG. 3B is a graph thatshows illustrative doping concentrations of portions of the bipolartransistor 300B of FIG. 3A. As shown in the legend 150 of FIG. 3F, thebipolar transistor 300B can be formed of substantially the samematerials as the bipolar transistor 100 and/or the bipolar transistor300A, with a different doping profile in the collector 120.

The bipolar transistor 300B can include a collector 120 having a firstcollector region 122 and an other collector region 335. The firstcollector region 122 can include any combination of features describedwith reference to the first collector region 122 of FIG. 1A. The othercollector region 335 can include a single grading in which dopingconcentration varies (for example, increases) away from the base 121 andspans the entire other collector region 335.

In order to meet RF gain specifications of a system, such as a poweramplifier system, that includes the bipolar transistor 300B, the singlegrading in the other collector region 335 of the bipolar transistor 300Bcan compensate for some or all of the losses in RF gain associated witha higher doping concentration in the first collector region 122. At thesame time, ACPR2 and/or ACLR2 specifications of a power amplifier systemthat includes the bipolar transistor 300B can still be met. The gradingin the other collector region 335 can increase BV_(CEX) and/or SOA ofthe bipolar transistor 330B. For instance, in certain embodiments, thedoping concentration in the other collector region 335 can have a dopingconcentration at the interface with the sub-collector 108 can beselected in the range from about 5×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³. The othercollector region 335 can have any suitable thickness or gradingdescribed herein to achieve one or more features described herein. Insome embodiments, the other collector region can have a thicknessselected from the range from about 4000 Å to 7000 Å. According tocertain embodiments, the grading in the other collector 335 can gradefrom about 7.5×10¹⁵ cm⁻³ at an interface with the first collector region122 to at least about 5×10¹⁶ cm⁻³ at an interface near or at thesub-collector 108.

FIG. 4 is an illustrative flow diagram of a process 400 of forming abipolar transistor according to an embodiment. It will be understoodthat any of the processes discussed herein may include greater or feweroperations and the operations may be performed in any order, asappropriate. Further, one or more acts of the process can be performedeither serially or in parallel. The process 400 can be performed whileforming the bipolar transistor 100 of FIG. 1A, the bipolar transistor300A of FIG. 3A, the bipolar transistor 300B of FIG. 3D, or anycombination thereof. At block 402, a sub-collector of a bipolartransistor is formed. The sub-collector can include any combination offeatures of the sub-collectors described herein, for example, thesub-collector 108. A collector region can be formed that includes atleast one grading at block 404. The at least one grading can be formedby any suitable doping method known in the art. The collector region canbe adjacent the sub-collector, for example, the directly over thesub-collector in the orientation of FIGS. 1A, 3A, and 3D. The collectorregion can include any combination of features described herein withreference to the other collector regions 125, 325, and/or 335. Forinstance, the collector region can have two gradings in someembodiments. The at least one grading of the collector region canincrease the RF gain of the bipolar transistor and/or increase theruggedness of the bipolar transistor. For example, the at least onegrading can compensate for some or all of the decrease in gain of thebipolar transistor that results from the high doping concentration inthe first collector region. A different collector region having a highdoping concentration can be formed abutting the base at block 406. Thehigh doping concentration can be any of the doping concentrations of thefirst collector region 122 described herein, for example, at least about3.0×10¹⁶ cm⁻³. Moreover, the high doping concentration and the thicknessof the first collector region can together improve one or more secondchannel linearity measures.

FIG. 5 is a schematic block diagram of a module 520 that can include oneor more bipolar transistors 100 of FIG. 1A, one or more bipolartransistors 300A of FIG. 3A, one or more bipolar transistors 300B ofFIG. 3D, or any combination thereof. The module 520 can be some or allof a power amplifier system. The module 520 can be referred to asmulti-chip module and/or a power amplifier module in someimplementations. The module 520 can include a substrate 522 (forexample, a packaging substrate), a die 524 (for example, a poweramplifier die), a matching network 525, the like, or any combinationthereof. Although not illustrated, the module 520 can include one ormore other dies and/or one or more circuit elements that coupled to thesubstrate 522 in some implementations. The one or more other dies caninclude, for example, a controller die, which can include a poweramplifier bias circuit and/or a direct current-to-direct current (DC-DC)converter. Example circuit element(s) mounted on the packaging substratecan include, for example, inductor(s), capacitor(s), impedance matchingnetwork(s), the like, or any combination thereof.

The module 520 can include a plurality of dies and/or other componentsmounted on and/or coupled to the substrate 522 of the module 520. Insome implementations, the substrate 522 can be a multi-layer substrateconfigured to support the dies and/or components and to provideelectrical connectivity to external circuitry when the module 520 ismounted on a circuit board, such as a phone board.

The power amplifier die 524 can receive a RF signal at an input pinRF_IN of the module 520. The power amplifier die 524 can include one ormore power amplifiers, including, for example, multi-stage poweramplifiers configured to amplify the RF signal. The power amplifier die524 can include an input matching network 530, a first stage poweramplifier 532 (which can be referred to as a driver amplifier (DA)), aninter-stage matching network 534, a second stage power amplifier 536(which can be referred to as an output amplifier (OA)), or anycombination thereof.

A power amplifier can include the first stage power amplifier 532 andthe second stage power amplifier 536. The first stage power amplifier532 and/or the second stage power amplifier 536 can include one or morebipolar transistors 100 of FIG. 1A, one or more bipolar transistors 300Aof FIG. 3A, one or more bipolar transistors 300B of FIG. 3D, or anycombination thereof. Moreover, the bipolar transistor 100 of FIG. 1A,the bipolar transistor 300A of FIG. 3A and/or the bipolar transistor300B of FIG. 3D can help meet the power module 520 and/or the poweramplifier die 524 to meet any of the linearity and/or RF gainspecifications described herein.

The RF input signal can be provided to the first stage power amplifier532 via the input matching network 530. The matching network 530 canreceive a first stage bias signal. The first bias signal can begenerated on the PA die 524, outside of the PA die 524 in the module520, or external to the module 520. The first stage power amplifier 532can amplify the RF input and provide the amplified RF input to thesecond stage power amplifier 536 via the inter-stage matching circuit534. The inter-stage matching circuit 534 can receive a second stagebias signal. The second stage bias signal can be generated on the PA die524, outside of the PA die 524 in the module 520, or external to themodule 520. The second stage power amplifier 536 can generate theamplified RF output signal.

The amplified RF output signal can be provided to an output pin RF_OUTof the power amplifier die 524 via an output matching network 525. Thematching network 525 can be provided on the module 520 to aid inreducing signal reflections and/or other signal distortions. The poweramplifier die 524 can be any suitable die. In some implementations, thepower amplifier 524 die is a gallium arsenide (GaAs) die. In some ofthese implementations, the GaAs die has transistors formed using aheterojunction bipolar transistor (HBT) process.

The module 520 can also include a one or more power supply pins, whichcan be electrically connected to, for example, the power amplifier die524. The one or more power supply pins can provide supply voltages tothe power amplifiers, such as V_(SUPPLY1) and V_(SUPPLY2), which canhave different voltage levels in some implementations. The module 520can include circuit element(s), such as inductor(s), which can beformed, for example, by a trace on the multi-chip module. Theinductor(s) can operate as a choke inductor, and can be disposed betweenthe supply voltage and the power amplifier die 524. In someimplementations, the inductor(s) are surface mounted. Additionally, thecircuit element(s) can include capacitor(s) electrically connected inparallel with the inductor(s) and configured to resonate at a frequencynear the frequency of a signal received on the pin RF_IN. In someimplementations, the capacitor(s) can include a surface mountedcapacitor.

The module 520 can be modified to include more or fewer components,including, for example, additional power amplifier dies, capacitorsand/or inductors. For instance, the module 520 can include one or moreadditional matching networks 525. As another example, the module 520 caninclude an additional power amplifier die, as well as an additionalcapacitor and inductor configured to operate as a parallel LC circuitdisposed between the additional power amplifier die and the power supplypin of the module 520. The module 520 can be configured to haveadditional pins, such as in implementations in which a separate powersupply is provided to an input stage disposed on the power amplifier die520 and/or implementations in which the module 520 operates over aplurality of bands.

The module 520 can have a low voltage positive bias supply of about 3.2V to 4.2 V, good linearity (for example, meeting any of the secondchannel linearity specification described herein), high efficiency (forexample, PAE of approximately 40% at 28.25 dBm), large dynamic range, asmall and low profile package (for example, 3 mm×3 mm×0.9 mm with a10-pad configuration), power down control, support low collector voltageoperation, digital enable, not require a reference voltage, CMOScompatible control signals, an integrated directional coupler, or anycombination thereof.

In some implementations, the module 520 is a power amplifier module thatis a fully matched 10-pad surface mount module developed for WidebandCode Division Multiple Access (WCDMA) applications. This small andefficient module can pack full 1920-1980 MHz bandwidth coverage into asingle compact package. Because of high efficiencies attained throughoutthe entire power range, the module 520 can deliver desirable talk-timeadvantages for mobile phones. The module 520 can meet the stringentspectral linearity requirements of High Speed Downlink Packet Access(HSDPA), High Speed Uplink Packet Access (HSUPA), and Long TermEvolution (LTE) data transmission with high power added efficiency. Adirectional coupler can be integrated into the module 520 and can thuseliminate the need for an external coupler.

The die 524 can be a power amplifier die embodied in a single GalliumArsenide (GaAs) Microwave Monolithic Integrated Circuit (MMIC) thatincludes all active circuitry of the module 520, such as one or more thebipolar transistors 100 of FIG. 1A, one or more bipolar transistors 300Aof FIG. 3A, one or more bipolar transistors 300B of FIG. 3D, or anycombination thereof. The MMIC can include on-board bias circuitry, aswell as input matching network 530 and inter-stage matching network 534.An output matching network 525 can have a 50 ohm load that is embodiedseparate from the die 524 within the package of the module 520 toincrease and/or optimize efficiency and power performance.

The module 520 can be manufactured with a GaAs Heterojunction BipolarTransistor (HBT) BiFET process that provides for all positive voltage DCsupply operation while maintaining high efficiency and good linearity(for example, meeting any of the second channel linearity specificationdescribed herein). Primary bias to the module 520 can be supplieddirectly or via an intermediate component from any three-cell Ni—Cdbattery, a single-cell Li-Ion battery, or other suitable battery with anoutput in the range selected from about 3.2 to 4.2 V. No referencevoltage is needed in some implementations. Power down can beaccomplished by setting an enable voltage to zero volts. No externalsupply side switch is needed as typical “off” leakage is a fewmicroamperes with full primary voltage supplied from the battery,according to some implementations.

Any of the devices, systems, methods, and apparatus described herein canbe implemented in a variety of electronic devices, such as a mobiledevice, which can also be referred to as a wireless device. FIG. 6 is aschematic block diagram of an example mobile device 601 that can includeone or more bipolar transistors 100 of FIG. 1A, one or more bipolartransistors 300A of FIG. 3A, one or more bipolar transistors 300B ofFIG. 3D, or any combination thereof.

Examples of the mobile device 601 can include, but are not limited to, acellular phone (for example, a smart phone), a laptop, a tabletcomputer, a personal digital assistant (PDA), an electronic book reader,and a portable digital media player. For instance, the mobile device 101can be a multi-band and/or multi-mode device such as amulti-band/multi-mode mobile phone configured to communicate using, forexample, Global System for Mobile (GSM), code division multiple access(CDMA), 3G, 4G, and/or long term evolution (LTE).

In certain embodiments, the mobile device 601 can include one or more ofa switching component 602, a transceiver component 603, an antenna 604,power amplifiers 605 that can include one or more bipolar transistors100 of FIG. 1A, one or more bipolar transistors 300A of FIG. 3A, one ormore bipolar transistors 300B of FIG. 3D, a control component 606, acomputer readable medium 607, a processor 608, a battery 609, and supplycontrol block 610.

The transceiver component 603 can generate RF signals for transmissionvia the antenna 604. Furthermore, the transceiver component 603 canreceive incoming RF signals from the antenna 604.

It will be understood that various functionalities associated with thetransmission and receiving of RF signals can be achieved by one or morecomponents that are collectively represented in FIG. 6 as thetransceiver 603. For example, a single component can be configured toprovide both transmitting and receiving functionalities. In anotherexample, transmitting and receiving functionalities can be provided byseparate components.

Similarly, it will be understood that various antenna functionalitiesassociated with the transmission and receiving of RF signals can beachieved by one or more components that are collectively represented inFIG. 6 as the antenna 604. For example, a single antenna can beconfigured to provide both transmitting and receiving functionalities.

In another example, transmitting and receiving functionalities can beprovided by separate antennas. In yet another example, different bandsassociated with the mobile device 601 can be provided with differentantennas.

In FIG. 6, one or more output signals from the transceiver 603 aredepicted as being provided to the antenna 604 via one or moretransmission paths. In the example shown, different transmission pathscan represent output paths associated with different bands and/ordifferent power outputs. For instance, the two example power amplifiers605 shown can represent amplifications associated with different poweroutput configurations (e.g., low power output and high power output),and/or amplifications associated with different bands.

In FIG. 6, one or more detected signals from the antenna 604 aredepicted as being provided to the transceiver 603 via one or morereceiving paths. In the example shown, different receiving paths canrepresent paths associated with different bands. For example, the fourexample paths shown can represent quad-band capability that some mobiledevices 601 are provided with.

To facilitate switching between receive and transmit paths, theswitching component 602 can be configured to electrically connect theantenna 604 to a selected transmit or receive path. Thus, the switchingcomponent 602 can provide a number of switching functionalitiesassociated with an operation of the mobile device 601. In certainembodiments, the switching component 602 can include a number ofswitches configured to provide functionalities associated with, forexample, switching between different bands, switching between differentpower modes, switching between transmission and receiving modes, or somecombination thereof. The switching component 602 can also be configuredto provide additional functionality, including filtering of signals. Forexample, the switching component 602 can include one or more duplexers.

The mobile device 601 can include one or more power amplifiers 605. RFpower amplifiers can be used to boost the power of a RF signal having arelatively low power. Thereafter, the boosted RF signal can be used fora variety of purposes, including driving the antenna of a transmitter.Power amplifiers 605 can be included in electronic devices, such asmobile phones, to amplify a RF signal for transmission. For example, inmobile phones having a an architecture for communicating under the 3Gand/or 4 G communications standards, a power amplifier can be used toamplify a RF signal. It can be desirable to manage the amplification ofthe RF signal, as a desired transmit power level can depend on how farthe user is away from a base station and/or the mobile environment.Power amplifiers can also be employed to aid in regulating the powerlevel of the RF signal over time, so as to prevent signal interferencefrom transmission during an assigned receive time slot. A poweramplifier module can include one or more power amplifiers.

FIG. 6 shows that in certain embodiments, a control component 606 can beprovided, and such a component can include circuitry configured toprovide various control functionalities associated with operations ofthe switching component 602, the power amplifiers 605, the supplycontrol 610, and/or other operating component(s).

In certain embodiments, a processor 608 can be configured to facilitateimplementation of various functionalities described herein. Computerprogram instructions associated with the operation of any of thecomponents described herein may be stored in a computer-readable memory607 that can direct the processor 608, such that the instructions storedin the computer-readable memory produce an article of manufactureincluding instructions which implement the various operating features ofthe mobile devices, modules, etc. described herein.

The illustrated mobile device 601 also includes the supply control block610, which can be used to provide a power supply to one or more poweramplifiers 605. For example, the supply control block 610 can include aDC-to-DC converter. However, in certain embodiments the supply controlblock 610 can include other blocks, such as, for example, an envelopetracker configured to vary the supply voltage provided to the poweramplifiers 605 based upon an envelope of the RF signal to be amplified.

The supply control block 610 can be electrically connected to thebattery 609, and the supply control block 610 can be configured to varythe voltage provided to the power amplifiers 605 based on an outputvoltage of a DC-DC converter. The battery 609 can be any suitablebattery for use in the mobile device 601, including, for example, alithium-ion battery. With at least one power amplifier 605 that includesone or more bipolar transistors 100 of FIG. 1A, one or more bipolartransistors 300A of FIG. 3A, one or more bipolar transistors 300B ofFIG. 3D, or any combination thereof, the power consumption of thebattery 109 can be reduced and/or the reliability of the power amplifier605 can be improved, thereby improving performance of the mobile device601.

Some of the embodiments described above have provided examples inconnection with modules and/or electronic devices that include poweramplifiers, such as mobile phones. However, the principles andadvantages of the embodiments can be used for any other systems orapparatus that have needs for a bipolar transistor with a high level ofsecond channel linearity without sacrificing RF gain.

Systems implementing one or more aspects of the present disclosure canbe implemented in various electronic devices. Examples of electronicdevices can include, but are not limited to, consumer electronicproducts, parts of the consumer electronic products, electronic testequipment, etc. More specifically, electronic devices configuredimplement one or more aspects of the present disclosure can include, butare not limited to, an RF transmitting device, any portable devicehaving a power amplifier, a mobile phone (for example, a smart phone), atelephone, a base station, a femtocell, a radar, a device configured tocommunication according to the WiFi and/or Bluetooth standards, atelevision, a computer monitor, a computer, a hand-held computer, atablet computer, a laptop computer, a personal digital assistant (PDA),a microwave, a refrigerator, an automobile, a stereo system, a DVDplayer, a CD player, a VCR, an MP3 player, a radio, a camcorder, acamera, a digital camera, a portable memory chip, a washer, a dryer, awasher/dryer, a copier, a facsimile machine, a scanner, a multifunctional peripheral device, a wrist watch, a clock, etc. Part of theconsumer electronic products can include a multi-chip module includingan RF transmission line, a power amplifier module, an integrated circuitincluding an RF transmission line, a substrate including an RFtransmission line, the like, or any combination thereof. Moreover, otherexamples of the electronic devices can also include, but are not limitedto, memory chips, memory modules, circuits of optical networks or othercommunication networks, and disk driver circuits. Further, theelectronic devices can include unfinished products.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The words “coupled,” “connected,” andthe like, as generally used herein, refers to two or more elements thatmay be either directly connected, or connected by way of one or moreintermediate elements. Additionally, the words “herein,” “above,”“below,” and words of similar import, when used in this application,shall refer to this application as a whole and not to any particularportions of this application. Where the context permits, words in theabove Detailed Description using the singular or plural number may alsoinclude the plural or singular number respectively. The word “or” inreference to a list of two or more items, that word covers all of thefollowing interpretations of the word: any of the items in the list, allof the items in the list, and any combination of the items in the list.All numerical values provided herein are intended to include similarvalues within a measurement error.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “e.g.,” “for example,” “such as” and the like,unless specifically stated otherwise, or otherwise understood within thecontext as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of embodiments is not intended to beexhaustive or to limit the invention to the precise form disclosedabove. While specific embodiments of, and examples for, the inventionare described above for illustrative purposes, For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having acts, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. For example, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. Moreover, theelements and acts of the various embodiments described above can becombined to provide further embodiments. Indeed, the methods, systems,apparatus, and articles of manufacture described herein may be embodiedin a variety of other forms; furthermore, various omissions,substitutions and changes in the form of the methods, systems,apparatus, and articles of manufacture described herein may be madewithout departing from the spirit of the disclosure. The accompanyingclaims and their equivalents are intended to cover such forms ormodifications as would fall within the scope and spirit of thedisclosure.

What is claimed is:
 1. A bipolar transistor comprising a collector, abase disposed over the collector, and an emitter, the collector having adoping concentration of at least about 3×10¹⁶ cm⁻³ in a first collectorregion abutting the base, the collector also having an other collectorregion under the first collector region, the other collector regionincluding at least one grading in which doping concentration increasesaway from the first collector region.
 2. The bipolar transistor of claim1 wherein the other collector region includes a first grading and asecond grading in which doping concentration increases away from thebase at a different rate than in the first grading.
 3. The bipolartransistor of claim 2 wherein the bipolar transistor has a gain of atleast about 29 dBm at a frequency within a frequency band centeredaround about 833 MHz.
 4. The bipolar transistor of claim 2 wherein thesecond grading is configured to increase Bv_(CEX) of the bipolartransistor compared to the same transistor without the second grading atthe same current density.
 5. The bipolar transistor of claim 2 wherein adoping concentration in the first grading grades from about an order ofmagnitude less than the doping concentration of the first collectorregion to less than the doping concentration of the first collectorregion.
 6. The bipolar transistor of claim 5 wherein a dopingconcentration in the second grading grades from about a maximum dopingconcentration in the first grading to a doping concentration that is atleast about one order of magnitude less than the doping concentration ofa sub-collector below the second grading.
 7. The bipolar transistor ofclaim 2 wherein the other collector region includes a second collectorregion proximate the first collector region and having a thickness thatis more than approximately twice the thickness of the first collectorregion, the first grading spanning the second collector region.
 8. Thebipolar transistor of claim 7 wherein the other collector region furtherincludes a third collector region having a thickness that is greaterthan the thickness of the first collector region and less than thethickness of the second collector region, the second grading spanningthe third collector region.
 9. The bipolar transistor of claim 8 whereinthe collector consists essentially of the first collector region, thesecond collector region, and the third collector region.
 10. The bipolartransistor of claim 9 further including a sub-collector under thecollector.
 11. The bipolar transistor of claim 2 wherein the firstgrading borders the second grading and doping concentration isapproximately the same on both sides of the border of the first gradingand the second grading.
 12. The bipolar transistor of claim 1 wherein athickness of the first collector region is selected from a range ofabout 1000 Å to 2000 Å.
 13. The bipolar transistor of claim 12 whereinthe doping concentration of the first collector region is selected froma range of about 3×10¹⁶ cm⁻³ to 9×10¹⁶ cm⁻³.
 14. The bipolar transistorof claim 1 wherein the doping concentration in the first collectorregion is at least about 6×10¹⁶ cm⁻³.
 15. The bipolar transistor ofclaim 1 wherein the base has a thickness of less than about 1400 Å. 16.The bipolar transistor of claim 15 wherein the base has a dopingconcentration selected from a range of about 3.5×10¹⁹ cm⁻³ to 7×10¹⁹cm⁻³.
 17. The bipolar transistor of claim 1 wherein the bipolartransistor is a heterojunction bipolar transistor (HBT).
 18. The bipolartransistor of claim 1 wherein the bipolar transistor is a GaAstransistor.
 19. A power amplifier module comprising a bipolar transistorhaving a collector, a base, and an emitter, the collector having adoping concentration at a junction with the base such that the poweramplifier has an alternative channel power ratio (ACPR2) of no greaterthan about −65 dBc, the collector also having at least a first gradingin which doping concentration increases away from the base.
 20. Thepower amplifier module of claim 19 wherein the ACPR2 is no greater thanabout −65 dBc when the power amplifier operates within a frequency bandcentered around approximately 833 MHz.
 21. The power amplifier module ofclaim 19 wherein the collector further includes a second grading fartherfrom the base than the first grading.
 22. The power amplifier system ofclaim 21 wherein the second grading is configured to increase Bv_(CEX)of the bipolar transistor compared to the same transistor without thesecond grading at the same current density.
 23. The power amplifiersystem of claim 19 wherein the doping concentration in the collector atthe junction with the base is at least about 3×10¹⁶ cm⁻³.
 24. The poweramplifier system of claim 19 wherein the collector includes a firstregion abutting the base having a substantially flat dopingconcentration of at least about 3×10¹⁶ cm⁻³ and a thickness selectedfrom a range of about 1000 Å to 2000 Å.
 25. The power amplifier systemof claim 24 wherein the doping concentration in the first region of thecollector is selected in the range from about 3×10¹⁶ cm⁻³ to 9×10¹⁶cm⁻³.
 26. A power amplifier die comprising a bipolar transistor having acollector, a base abutting the collector, and an emitter, the collectorhaving a doping concentration of at least about 3×10¹⁶ cm⁻³ at ajunction with the base, the collector also having at least a firstgrading in which doping concentration increases away from the base. 27.A mobile device comprising an antenna, a battery, and a power amplifierincluding a heterojunction bipolar transistor having a collector, abase, and an emitter, the collector including a first collector regionabutting the base and having a first doping concentration of at leastabout 3×10¹⁶ cm⁻³, a second collector region proximate the firstcollector region and having a first grading in which dopingconcentration increases away from the base, and a third collector regionproximate the second collector region and having a second grading inwhich doping concentration increases away from the base at a differentrate than the first grading, the first doping concentration, the firstgrading, and the second grading configured to improve linearity of thepower amplifier.
 28. A method of forming a bipolar transistor, themethod comprising: forming a sub-collector; forming a collector regionwith at least one grading having a doping concentration that decreasesaway from the sub-collector; and forming a different collector regionadjacent abutting a base of the bipolar transistor and having a dopingconcentration of at least about 3×10¹⁶ cm⁻³ at an interface with thebase.